a) This paper is based on a talk presented by the authors at the 28th International Conference on the Physics of Semiconductors, which was held 24-28 July 2006, in Vienna, Austria. Contributed papers for that conference may be found in “Physics of Semiconductors: 28th International Conference on the Physics of Semiconductors,” AIP Conference Proceedings No. 893 (AIP, Melville, NY, 2007); see http://proceedings.aip.org/proceedings/confproceed/893.jsp

Abstract

We review recent studies by optics methods of emergent phases in the quantum Hall (QH) regimes of double layers with finite tunneling at Landau level filling factor . In measurements of spin excitations by inelastic light scattering and of elastically scattered Rayleigh light under the application of in-plane magnetic fields, we uncovered evidence of a quantum phase transition that occurs when a many-body tunneling gap collapses. The transformation can be regarded as a transition from an incompressible highly correlated QH state to a compressible composite-fermion bilayer system. The correlated QH state is characterized by the presence of populations of bound electron-hole pairs across the tunneling gap. Quantitative determinations of the density of such excitonic pairs are obtained from inelastic light scattering spectra of spin excitations. The correlated QH state displays resonant Rayleigh scattering with unusual temperature dependence.

Received 24 July 2006Accepted 25 October 2006Published online 27 April 2007

Acknowledgments:

V.P. acknowledges support from the Italian Ministry of Foreign Affairs, Italian Ministry of Research, and European Community’s Human Potential Program (Project No. HPRN-CT-2002-00291). A.P. acknowledges support by the National Science Foundation under Award No. DMR-03-52738, by the Department of Energy under Award No. DE-AIO2-04ER46133, by the Nanoscale Science and Engineering Initiative of the National Science Foundation under Award No. CHE-0117752, and by a research grant of the W. M. Keck Foundation.